Arc lamp and method



June 14, 1966 c. H. KELLER ETAL 3,256,459

ARC LAMP AND METHOD Filed Feb. 15, 1963 2 Sheets-Sheet 1 JNVENTORS CHARLES H. KELLER BY RAYMOND E. PAQUETTE UM, LMLWF ATTORNEY June 14, 19766 c. H. KELLER ETAL 3,256,459

ARC LAMP AND METHOD 2 Sheets-Sheet 2 Filed Feb. 15, 1963 DISTANCE INVENTORS CHARLES H. KELLER BY RAYMOND E. PAQUETTE M MMF ATTORNEY requiring very intense sources of light.

United States Patent 3,256,459 ARC LAMP AND METHOD Charles H. Keller and Raymond E. Paquette, Sunnyvale, Calif., assignors to Pek Labs, Inc., Sunnyvale, Calif., a

corporation of California Filed Feb. 15, 1963, Ser. No. 258,717 12 Claims. (Cl. 31434) high voltage starting pulse.

Arc light sources, such as positive pressure short arc lamps, are in relatively common use today in applications For example, such light sources find application for oscillographic photography, for solar simulation to determine deterioration due to sunlight, for search lights and the like. To start such lamps requires the initial application of a very high voltage starting pulse, typically 16,000 volts, supplied by a starting or trigger pulse power supply, to partially ionize the gas between the electrodes to flash the arc. Once started, the arc is maintained by the application of power at low voltage, typically around 12 volts provided by a sustaining or main power supply.

Since arc lamps usually support their electrodes within the stems of the pressurized glass or quartz envelope, and since current leads connected to the electrodes must be passed through the envelope and therefore must be very thin to facilitate sealing to the envelope, the energy applied by the starting pulse power supply must be carefully limited to prevent the generation of an excessive starting current which burns out the leads, usually referred to as ribbons. Typically, the energy of the starting pulse is limited to 2 watt-seconds. In addition to requiring a starting pulse power supply for supplying the high voltage starting pulse having a selected minimum voltage and a selected maximum energy, means must be provided for inter-coupling the starting pulse power supply with the main power supply which means must pro-' tect themain power supply and the arc lamp from being damaged or burned out by the starting pulse.

Starting pulse power supplies typically include a capacitorwhich is charged to the required starting pulse voltage and which has a storage capacity selected not to exceed the maximum permissible energy which the starting pulse may have. The starting pulse is prevented from reaching the main power supply by suitable filtering means. The necessary high voltage starting pulse may also be developed by an alternating current generator coupled by means of a first'transformer to a spark gap circuit which in turn is coupled, by a second transformer, to an output circuit connected across the lamp electrodes. It is therefore seen that the necessity of providing a high voltage starting pulse requires the addition of much circuitry and equipment whose sole function is to start the arc and which is not needed to sustain the same.

In many applications, where it is desirable to provide an intense source of light, the weight, complexity, size and expense associated with the starting pulse power supply have been found a serious limitation on their use. For example, the use of arc lamps as intense light sources to which a modulated sustaining current may be applied for excellent discrimination against the sun have found little application so far as an optical tracking element in airborne vehicles because of the necessity to furnish a starting pulse power supply.

It is therefore a primary object of this invention to provide a positive pressure are lamp which is started by connection to a main power supply without use of a high voltage starting pulse.

It is another object of this invention to provide an arc lamp which is so constructed that the application of power, normally designed to sustain the arc, also starts the arc.

It is a further object .of this invention to provide a short are lamp capable of being started and sustained, in oneshot operations, by connecting to a main power supply.

It is a still further object of this invention to provide a short are lamp, particularly suited as the source of illumination for the optical tracking of an airborne vehicle, which is so constructed that it may be operated without the application of a high voltage starting pulse.

It is a still further object of this invention to provide an arc lamp which may be operated with a minimum of auxiliary circuitry and power sources and which therefore results in an arc lamp system which is lighter, smaller, more reliable and more economical than has been known heretofore.

It is also an object of this invention to provide a new and novel method of starting an arc lamp which offers great advantages where small size, low weight and high reliability are important factors.

Briefly, the arc lamp of this invention makes use of a conductive fusion bridge spanning the gap between the lamp main electrodes in the lamp envelope to thereby form a low resistance electrical path therebetween. The fusion bridge is so shaped and cross sectioned that application of the low voltage from the main power supply will vaporize this bridge progressively to thereby increase the effective electrode spacing until the arc is operating in its normal mode between the two main electrodes. The progressive vaporization of the fusion bridge is analogous to physically withdrawing one of two touching electrodes until the proper spacing is achieved for'normal arc operation.

Other objects and a better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a view, partly in elevational and partly in cross section with some parts broken away, of an arc lamp embodying the structural features of this invention;

FIG. 2 is an enlarged view, partly in elevation and partly in cross section with some parts broken away, of the main electrodes, fusion bridge and bridge support of the arc lamp shown in FIG. 1;

FIG. 3 is a view taken along line 3-3 of FIG. 2;

FIG. 4 is an illustrative diagram useful in the explanation of the operation of this invention; and

FIGS. 5, 6 and 7 are'elevational and perspective views of different embodiments of the fusion bridge spanning thegap between the main electrodes as shown in FIG. 2.

As is well known in this art, xenon short are lamps comprise generally an anode and a cathode sealed in a quartz envelope containing a relatively high pressure xenon atmosphere which, at room temperature, is typically from between 3 to 7 atmospheres which corresponds to about 15 to 30 atmospheres at operating temperatures. The anode and cathode are separated by a relatively short distance and produce an intense arc, started in prior art devices by the high voltage starting pulse from a high voltage power supply and sustained by thelow voltage from a main power supply.

Referring now to the drawing, and particularly to FIGS. 1 to 3 thereof, there is shown a short are lamp 10 constructed in accordance with this invention. Lamp 10 includes a pressurized envelope 12 which for structural strength is preferably of spherical form-and which is constructed of quartz for well understood reasons. Envelope 12 is provided with axially aligned stems, shown as 14 and 16. i

Mounted in stem 14 is an anode 18 of refractory metal,

usually swaged tungsten. As illustrated, it comprises a rod-like portion mounted in stem 14 and terminating at the arc end in an enlarged cylindrical portion 22. The parts 18 and 20 are commonly integral and the tungsten is worked to the form illustrated. The end of portion 20 is Welded to a thin conductive ribbon 24 which is sealed in stem 14 and which provides a current lead connection. Ribbon 24 is electrically connected to a lamp connector 26 to which external connections are made.

Mounted in stem 16 is a cathode 28 of refractory metal similar to that used for anode 18. Also, cathode 28 consists of a rod-like portion 30 mounted in stem 16 and terminating at the arc end into an enlarged cylindrical portion 32 which is smaller than portion 22. As before, parts 30 and 32 are usually integral and end portion of 30 is welded to a thin conductive ribbon 34 which is sealed in the end of stem 16 and which serves as a current lead thereto. Ribbon 34 is electrically connected to a metallic connector 36 to which external connections are made.

Cathode 28 is axially aligned with anode 18 and its cylindrical portion 32 has a diameter smaller than that of cylindrical portion 22 for well understood reasons. Also, cylindrical portion 32 has wound around it a coil 38 which acts to regulate heat dissipation from the cathode and to contain cathode conditioning materials to aid in stabilizing the are at the cathode arc end. The are end of both anode 18 and cathode 28 are shaped to define cones pointed towards one another to facilitate the formation of the arc therebetween and once formed, to maintain the arc in a definite position and to prevent the arc ends from wandering over the electrodes.

Spanning the gap between the arc ends of anode 18 and cathode 28, and in electrical contact therewith, is a fusion bridge 40. Bridge 4t] is mechanically supported by a wire-like support element 42 sealed to envelope 12. As a practical matter, support element 42 provides means of positioning bridge 40 during assembly of the arc lamp of this invention and, after properly positioning, element I 42 is sealed to envelope 12.

The term fusion bridge, as used herein, is defined as a conductive member of sufficient length to span the gap for conductive contact with opposite electrodes of an arc lamp. Also, it is of such cross section in the direction of current flow and comprised of such a material that the application of normal lamp operating power fuses (burns) the bridge in a short time which is usually selected to be sufficiently long so that the arc can follow and sufficiently short to establish the are without undue delay. Typical burn out times are between microseconds and 0.5 second with a preferred burn-out time of about 0.1 second.

A typical xenon short are lamp pressurized to 6 atmospheres of xenon gas and having an arc gap of approximately /2 millimeter between electrodes may be designed to dissipate approximately 75 watts, at 13 volts and 5.8 amperes. In order to accommodate lamps with normal manufacturing variations in arc spacing, the typical lamp power supply is normally designed such that E-I curve of the power supply is approximately tangent to the locus of constant wattage line at the design center value of voltage and current, and delivers less than the short circuit current which would damage the lamp. These design criteria result in a power supply which acts as an approximate constant wattage supply for all lamps having characteristics close to the design center.

than 75 watts at say a starting current of 10 amperes and a final current of say 6 amperes completely disintegrates bridge 40 in about 0.1 second. For example, a triangularly shaped fusion bridge of molybdenum of flat stock having a thickness of about one-half thousandths of an inch has a burn-out time of approximately 0.1 of a second.

As a result of the application of this current, the fusion bridge heats up rapidly and soon reaches its melting point at which time fusion commences. In case of the triangular-shaped bridge 40, the temperature distribution prior to the commencement of fusion is shown by curve 50, FIG. 4. Curve 50 is a plot of position as abscissa superimposed upon the gap region of a lamp with'a fusion bridge 40, against temperature as ordinate. Line 52 indicates the melting temperature of the material of fusion bridge 48. As is readily seen, the temperature is greatest where the cross section is smallest, namely at the apex of the triangle joined to cathode 28 where the current density is greatest. Consequently, fusion occurs there and continues progressively to the right until bridge 40 is entirely consumed. During such progressive consumation, the arc isdrawn from cathode 32, at a rate depending on the rate of consumation, to anode 22.

The material of which fusion bridge 40 is formed is not critical and may be selected on the basis of melting point and conductivity. However, in certain applications, particularly where the arc is to provide a radiation of say the xenon spectrum, it has been found desirable to utilize a material which does not change the properties of pressurizing gas. For example, with xenon lamp a fusion bridge of molybdenum has been found not to effect the spectral radiation materially, a property which may be important for certain applications of optical tracking where high discrimination against other light sources is desired.

Referring now to FIGS. 5, 6 and 7, there are shown different embodiments of fusion bridges with which this invention may be practiced. For example, FIG. 5 shows an anode 60, a cathode 62 surrounded by a coil 64 which defines a gap with the anode which is spanned by a fusion bridge 66 of conical shape. As before, because of the progressively decreasing cross section along the length of bridge 66, the current density is greatest at the apex of the cone-shaped bridge which therefore will reach its melting temperature first and bridge 66 will progressively melt towards anode 60.

FIG. 6 shows a gap defined between an anode 68 and a cathode 70 having a coil 72 wound about its outer pcriphery. The gap is bridged by a fusion bridge 74 of cylindrical shape with a conical end portion contacting cathode 70. Again, since the current per unit cross section is greatest at the conical end portion of bridge 74, the same will burn progressively towards anode 68. Since in this embodiment a major portion of bridge 74 is of uniform cross section it is possible that a large length of bridge 74 may reach the melting temperature at the same time and cause the gap to enlarge very rapidly. However, it has been found that the are usually follows sufficiently rapidly to start the arc. This situation may be analogized with the slow mechanical withdrawal of one electrode from the other to establish the arc and then a rapid widening of the gap to the proper electrode spacing.

FIG. 7 shows still another embodiment of this invention in which the gap between an anode 76 and a cathode 78, having a coil 80, is spanned by a fusion bridge 84 shaped in the form of a flat member cut to resemble a pair of isosceles trapezoids having a common narrow top (placed end-to-end). The opposite wide base lines may be provided with V-shaped cutouts to fit over the conical arc ends of electrodes 76 and 78. It is interesting to note that bridge 84 is consumed from the center, where the constriction is located, and then outwardly so that the gap is enlarged substantially uniformly towards both electrodes simultaneously.

In operation, the main power supply is connected across lamp connectors 26 and 36 which immediately initiates maximum current flow since the electrode gap is bridged by a conductive fusion bridge. The fusion bridge is preferably of progressively changing across section so that it progressively disintegrates as the temperature passes the melting point of the bridge. The progressive disintegration of the fusion bridge increases the disintegrated gap drawing the arc progressively from the cathode to the anode in the same manner as a mechanically movable anode would do.

It will be understood that the embodiments illustrated in FIGS. 5, 6 and 7 are but three examples of many pos sible bridge configurations and that many others will become obvious in view of the teachings made herein.

There has been described a self starting are lamp which together with the arc initiating and maintaining system is light, small, economical and reliable. The arc lamp of this invention is constructed for one shot operation 'much in the same manner as airborne vehicles in which it finds an important application.

What is claimed is:

1. The method of starting and operating an arc lamp, including a pressurized envelop and a pair of spaced apart electrodes immovably disposed within said envelop having arc ends defining an arc gap therebetween, said method comprising the steps: conductively spanning said are gap with a fusion bridge which melts when subjected to normal arc lamp sustaining power; and applying normal arc lamp sustaining power across said electrodes to melt said fusion bridge to first establish the arc and to thereafter sustain the once established are.

2. In a short are lamp, including a pressure envelop and a pair of spaced apart electrodes immovably disposed Within the envelop which have are ends defining an arc gap therebetween, the improvement in the form of a self starting feature comprising: a conductive fusion bridge conductively connected across the arc ends of said electrodes to bridge said are gap, said fusion bridge being dimensioned so that the electrical energy, normally applied across said electrode to sustain a once established arc, consumes said fusion bridge by melting to establish the arc.

3. In a short are lamp having a pair of spaced apart electrodes, defining a gap therebetween, immovably disposed within a pressurized envelop, the improvement in the form of a self starting feature comprising: a consumable conductive fusion bridge conductively connected across the arc ends of said electrodes and bridging said gap, the cross-sectional area of at least one portion of said' fusion bridge being sufiiciently small so that the electrcal energy, normally applied to the short are lamp to sustain a once established are between said electrodes,

.melts said fusion bridge at said portion to thereby establish an arc, the cross-sectional area of said fusion bridge increasing along its direction of extension on either side of said portion.

4. In the combinationtof an arc lamp in which a pair of spaced apart electrodes are disposed within a pressurized envelope todefine a gap of predetermined length therebetween, and a low voltage power supply having a power output commensurate with sustaining a once established are which is connectible across the electrodes, the self starting feature in the arc lamp comprising: a consumable conductive fusion bridge conductively connected to the arc ends of said electrodes and bridging said gap, said fusion bridge being comprised of a conductive material and having a minimum cross sectional area at least at some portion along the direction of electrode current flow such that application of electric power from power supply raises its temperature at said portion above its melting point to initiate an arc which progressively 6 widens by progressively consuming said bridge until the arc operates across the arc ends of said electrodes.

5. An arc lamp comprising:

a sealed envelope having disposed therein an anode and a cathode defining a gap therebetween; separate electrical conductormeans connected to said anode and cathode and passing through said envelope for connection with a source of electrical energy; and

a consumable conductive fusion bridge within said envelope and disposed in said gap and in electrical contact with said anode and said cathode, said fusion bridge having a minimum cross sectional area at least at some portion along its length which is consumable by the electrical energy normally required to maintain a once established arcacross the gap in the arc lamp.

6. An arc lamp in accordance with claim 5 which further includes an envelope supported mounting means for supporting said fusion bridge between said anode and said cathode.

7. An arc lamp in accordance with claim 5 in which the cross sectional area of said fusion bridge increases progressively along at least a portion of its length to one side of said minimum cross sectional area.

8. An arc lamp in accordance with claim 5 in which said fusion bridge is of fiat stock and triangular in shape.

9. An arc lamp in accordance with claim 5 in which said fusion bridge is conical in form and mounted so that the vertex of the cone is touching said cathode.

10. An arc lamp in accordance with claim 5 in which said fusion bridge is substantially cylindrical in form having a conically shaped end portion in contact with said cathode.

'11. An arc lamp in accordance with claim 5 in which the cross sectional area of said fusion bridge progressively increases along at least a portion of its length to either side of said minimum cross sectional area.

12. A one-shot, self starting arc lamp system comprising in combination:

a short are lamp including a pressurized envelope and a pair of spaced apart electrodes immovably disposed within said envelope and defining an arc gap therebetween;

a power supply connectible across said electrodes for starting and thereafter operating said short are lamp, said power supply having an electrical voltage and Wattage output which is commensurate with the voltage and wattage required to maintain a once established are said gap; and

a consumable conductive fusion bridge Within said enevelope and disposed across said gap and in elec trical contact with said electrodes, said fusion bridge having a minimum cross sectional area at least at some portion along its length which fuses when said power supply is connected across said electrodes to thereby establish and thercaftermaintain an arc across said gap.

References Cited by the Examiner UNITED STATES PATENTS RICHARD M. WOOD, Primary Examiner. JOSEPH V. TRUHE, Examiner, 

1. THE METHOD OF STARTING AND OPERATING AN ARC LAMP, INCLUDING A PRESSURIZED ENVELOPE AND A PAIR OF SPACED APART ELECTRODES IMMOVABLY DISPOSED WITHIN SAID ENVELOPE HAVING ARC END DEFINING AN ARC GAP THEREBETWEEN, SAID METHOD COMPRISING THE STEPS: CONDUCTIVELY SPANNING SAID ARC GAP WITH A FUSION BRIDGE WHICH MELTS WHEN SUBJECTED TO NORMAL ARC LAMP SUSTAINING POWER; AND APPLYING NORMAL ARC LAMP SUSTAINING POWER ACROSS SAID ELECTRODES TO MELT SAID FUSION BRIDGE TO FIRST ESTABLISH THE ARC AND TO THEREAFTER SUSTAIN THE ONE ESTABLISHED ARC. 